Fast, rapid (6)
I believe the answer is:
speedy
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">fast</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">rapid</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that rapid can be written as speedy)</span><br/ ><br/ ></span>
(Other definitions for speedy that I've seen before include "Rapid, fast" , "Fast-moving" , "in a hurry" , "Done quickly" , "in a fast car?" .)